1. Field of the Invention
The present invention relates to a high electron mobility transistor for use in a radio-frequency amplifier which operates in a millimeter wave band, and a method of manufacturing such a high electron mobility transistor and a field-effect transistor.
2. Description of the Prior Art
There has been developed a high electron mobility transistor which has a channel layer of high electron mobility (aluminum gallium arsenide) for improved radio-frequency characteristics. In the developed high electron mobility transistor, a thin layer of InGaAs is formed intermediate in a wide-band gap layer of high-resistance AlGaAs (aluminum gallium arsenide), thereby producing a channel layer of double heterojunction structure. Electrons are supplied at a relatively high concentration into the channel layer from silicon-doped planar layers that are disposed respectively in upper and lower wide-band gap layer portions.
U.S. patent application Ser. No. 08/565,295 filed Nov. 30, 1995, entitled "FIELD-EFFECT TRANSISTOR" and assigned to the present assignee, discloses a high-performance high electron mobility transistor whose mutual conductance changes to a small degree with respect to a gate voltage. because the thickness of the channel layer is limited to a value small enough to regard an electron gas layer as a substantially single layer and the upper and lower wide-band gap layer portions of AlGaAs have a high resistance.
If the channel layer is thick, the electron gas layer formed in the channel layer is localized in the vicinity of the heterojunction plane, and hence separated into two layers whose depths from the transistor surface are different from each other, i.e., whose distances from the gate electrode are different from each other. The gate voltage has different effects on two electron (gas layers which are separated at different depths, i.e., spaced different distances from the gate electrode. As a consequence, the mutual conductance becomes largely dependent on the gate voltage.
According to the invention disclosed in U.S. patent application Ser. No. 08/565295 referred to above, the thickness of the channel layer is limited to a value small enough to regard an electron gas layer as a substantially single layer, specifically to a thickness in the range of from 50 .ANG.to 150 .ANG., and the upper and lower layers of AlGaAs disposed adjacent to the thin channel layer have a high resistance. The upper and lower layers of AlGaAs above and below the thin channel layer have a high resistance because as the resistance of the upper and lower layers of AlGaAs increases, the gate voltage affects a wider area including the channel layer, resulting in the same effect as caused by.a reduction in the thickness of the channel layer.
FIG. 1 of the accompanying drawings schematically shows the structure of the high electron mobility transistor disclosed in U.S. patent application Ser. No. 08/565,295 referred to above. As shown in FIG. 1, the high electron mobility transistor has a semi-insulating GaAs substrate 31, a super-lattice buffer layer 32 disposed on the semi-insulating GaAs substrate 31 for preventing an unwanted carrier from leaking, a pair of lower and upper wide-band gap layers 33, 35 of AlGaAs disposed on the super-lattice buffer layer 32, an InGaAs channel layer 34 disposed between the lower and upper wide-band gap layers 33, 35, a pair of silicon-doped planar layers 33a, 35a disposed respectively in the lower and upper wide-band gap layers 33, 35, an n.sup.+ GaAs contact layer 36 disposed on the upper wide-band gap layer 35, an SiO.sub.2 film 37 disposed on the n.sup.+ GaAs contact layer 365, a passivation film 38 disposed on the SiO.sub.2 film 37, and a gate electrode 39 of T-shaped cross section disposed on the upper wide-band gap layer 35 and covered with the passivation film 38.
The channel layer 34 has a thickness which is selected to be of a value small enough to cause electron gases, which would otherwise be localized in the vicinity of heterojunctions formed between the channel layer 34 and the lower and upper wide-band gap layers 33, 35 and hence tend to be separated from each other, to be combined into a single electron gas layer that is controllable depending on changes in the gate electrode. Specifically, the thickness of the channel layer 34 is in the range of from 50 .ANG. to 150 .ANG..
FIG. 2 of the accompanying drawings shows experimental data on electric characteristics of the high electron mobility transistor shown in FIG. 1. The electric characteristics shown in FIG. 2 represent the relationship between the drain voltage and the drain current at various discrete values of the gate voltage. It can be seen from FIG. 2 that the drain current increases substantially uniformly as the gate voltage increases and the mutual conductance does not vary greatly depending on the gate electrode.
FIG. 3 of the accompanying drawings shows experimental data on the relationship between the mutual conductance and the gate voltage of the high electron mobility transistor or field-effect transistor (FET) shown in FIG. 1, a conventional high electron mobility transistor or FET, and an improved conventional high electron mobility transistor or FET. The graph shown in FIG. 3 has a vertical axis indicative of the mutual conductance (gm) per unit gate width and a horizontal axis indicative of the gate voltage (V). In FIG. 3, the solid-line curve A represents the experimental data of the high electron mobility transistor shown in FIG. 1, the broken-line curve B represents the experimental data of the conventional high electron mobility transistor, and the dot-and-dash-line curve C represents the experimental data of the improved conventional high electron mobility transistor. The experimental results illustrated in FIG. 3 clearly shows justifying support for the advantages of the, high electron mobility transistor disclosed in U.S. patent application Ser. No. 08/565,295.
U.S. patent application Ser. No. 08/558,548 filed Nov. 15, 1995 discloses a method of manufacturing a semiconductor device, which is suitable for the manufacture of the high electron mobility transistor shown in FIG. 1. According to the disclosed method, an etching solution suitable for a selective wet etching process for forming a recess in which a gate electrode will be formed comprises a mixture of ammonia water and hydrogen peroxide water mixed at a ratio of 1 to 4000 or more and diluted by water.
In the high electron mobility transistor shown in FIG. 1, the entire surface of the T-shaped gate electrode 39 is covered with the passivation film 38 . The passivation film 38 comprises an SiO.sub.2 /N film or the like having a large dielectric constant which is several times the dielectric constant of air. Therefore, the parasitic capacitance of the gate electrode 39 is so large that the high electron mobility transistor has poor radio-frecluency characteristics.